A method of this kind and a device are, for example, known from the publication "Methods of Enzymology", Vol. 193, pp. 280 (1990) by F. Hillenkamp and M. Karas or also from M. Karas and F. Hillenkamp, Anal. Chem. 60, 2299 (1988).
The time-of-flight mass spectrometer (TOF=time-of-flight) is based on measurements of the ions time of flight. In addition to other effects, the mass resolution for an instrument of this kind is limited by time smearing of the signal by the ion detector during detection. For this reason, so-called microchannel plate detectors are customarily used. These typically consist of microchannels lying side by side with a diameter of approximately 10 .mu.m. These channels are arranged at an angle of approximately 10.degree. to the surface normal. An arrangement of this kind provides a detection-sensitive surface which is flat except for low penetration depth and aligned vertically to the ion beam so that practically no differences in the time of flight arise before detection. In addition, the design is very short (typically 0.5 mm) so that the total time of flight of the converted electrons is extremely brief and so that the time smearing is also very small. It has been possible to measure peak widths of &lt;2.5 ns in a time-of-flight mass spectrometer with detectors of this kind (K. Walter, U. Boesl and E. W. Schlag, Int. Journ. Mass Spec. Ion Procs. 71 (1986) 309-313).
The detection of ions in a microchannel plate detector is based on the fact that the ions are "converted" into electrons on falling onto the surface of the detector and that these are then therefore "amplified" in the microchannels, i.e. multiplied as in a usual secondary-emission multiplier.
Since the introduction of matrix-assisted laser desorption ("MALD"; M. Karas and F. Hillenkamp, Anal. Chem. 60, (1988) 2299; K. Tanaka et al. Rapid Commun. Mass Spectrom. 2, (1988) 151) as a technique for generating ions with a very large mass-to-charge ratio (m/q), there has been a tremendous increase in interest in the effective detection of ions in the mass range with m/q up to 500,000 and above.
The microchannel plate detector has, however, two major disadvantages for this application:
1. It can be easily saturated. With a great amount of signal in a small mass range (approx. 20,000 to 200,000 ions/cm.sup.2), e.g. from matrix ions, from much chemical background or from polymers, in which ions are distributed over a very wide mass range, the detection sensitivity for very large masses sinks to zero.
2. Large molecules more readily generate secondary ions instead of secondary electrons. For larger masses, the probability to convert to e.sup.- drops and becomes very small, as demonstrated by J. Martens, W. Ens and K. G. Standing in "Proceedings of the ASMS 1991" with a mass of 66,000 u. Instead of e.sup.-, both positive and negative secondary ions are readily generated. Our own examinations have shown that particularly the negative secondary ions of negative primary ions provide clearly improved signals in the detection of negative polymer ions of high molecular weight.
Until now the disadvantages stated above have either been accepted, or a secondary-emission multiplier with a first dynode some distance away was used, on which secondary ions are produced by conversion which are then accelerated onto the second dynode, there generating electrons which are afterwards amplified in a multiplier as usual. In contrast to a standard secondary-emission multiplier, a voltage of several kilovolts is applied between the first and second dynodes to enable the secondary ions produced at the first dynode to receive sufficient energy to generate secondary electrons on falling onto the second dynode in the papers quoted above, matrix-assisted laser desorption was carried out with a detector of this kind. It has a good sensitivity for molecules with large m/z owing to the conversion into small secondary ions at the first dynode and is more insensitive to saturation than a microchannel plate detector. The disadvantages are as follows:
1. The time resolution of the detector and thus the mass resolution of the mass spectrometer are poor. There are two reasons for this: a) The usual dynodes used (Venetian blind type) have a thickness of typically 4 mm. Depending on the point at which the ions fall onto the dynodes arranged at approx. 45.degree., the flight route is up to 4 mm longer. With a flight tube length of 1 m this results in a time inaccuracy (dt) of 0.4% of the total time (T). The resolution (R) is defined as R=T/2dt, this fact limiting the resolution to R&lt;125. b) The secondary ions generated at the conversion dynode are accelerated onto the next dynode which also has a thickness of 4 mm. This again results in a time smearing in the detection of the secondary ions. In addition, the secondary ions have a mass distribution from mass I (H.sup.-) to approx. mass 100 (B. Spengler et al., Proceedings of the 38th ASMS Conference on Mass Spectrometry and Allied Topics (1990), pp. 162). This results in a further time smearing since the small secondary ions are accelerated more quickly onto the next dynode.
2. The detection of negative secondary ions is not possible since the uppermost dynodes have negative high voltage, e.g. -3 kV, if the signal output and thus the further amplifier electronics are required to have ground potential.
Therefore, it is among the objects of the invention to develop a method of detection and a detector for a time-of-flight mass spectrometer which is still suitable for large molecules (m&gt;10,000 u) and enables the detection of positive and negative secondary ions.